Search for Galactic Cosmic Ray Sources with H.E.S.S

0.08 MB·English

by  Nukri Komin

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0 Search for Galactic Cosmic Ray Sources with H.E.S.S.
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n
a Nukri Komin on behalf of the H.E.S.S. collaboration
J
LAPP, CNRS/IN2P3, 9 Chemin de Bellevue,

3 74941 Annecy-le-Vieux, France

]
E Supernova remnants (SNRs) are the prime candidates for the acceleration of the Galactic
H CosmicRays. TracersforinteractionsofCosmicRayswithambientmaterialaregammarays
atTeVenergies,whichcanbeobservedwithgroundbasedCherenkovtelescopeslikeH.E.S.S.
.
h IntherecentyearsH.E.S.S.hasdetectedseveralSNRsandinteractionsofSNRswithmolecular
p clouds. Herethecurrentresultsoftheseobservationsarepresentedandpossible leptonicand
-
hadronic scenarios are discussed. It is shown that it is likely that SNRs are the sources of
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r Galactic Cosmic Rays.
t
s
a
[ 1 Introduction

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v Since the discovery of the Cosmic Rays in 1912 their origin remains a mystery. Very promising
1 candidates for acceleration of at least the Galactic Cosmic Rays (up to ≈ 1015eV) are SNRs; it
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6 has been shown that several percent of theenergy of every supernovaexplosion (of the order of
0 1051erg) can easily supply the entire power found in the Cosmic Rays. The theory of Diffusive
.
1 Shock Acceleration (DSA)2 shows that particles can be accelerated up to relativistic energies
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in shock fronts. Among others, these shock fronts can be found in the shells of SNRs.
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1 As Cosmic Rays as charged particles are deflected in magnetic fields their direct observation
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v does not carry any information on their origin. Observations of photons producedby relativistic
i particles are much more promising messengers. The discovery of synchrotron emission from
X
SNRs shows that at least electrons and positrons are accelerated in the shells of SNRs. A tracer
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a for acceleration of protons are gamma rays from hadronic interactions of relativistic protons
with ambient material. The resulting photon spectrum is relatively flat and ranges from MeV
to TeV energies. These gamma-rays can be observed by satellite experiments in the MeV and
GeV energy range (e.g. by the Fermi/LAT) or with ground based telescopes above ≈ 100GeV.
The High Energy Stereoscopic System (H.E.S.S.) is a Cherenkov telescope array located in
Namibia for the observation of very heigh energy (VHE) gamma rays (≈ 100,GeV up to several
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tens of TeV) . Due to its location in the southern hemisphere it can observe a large part of the
Galactic plane. Stereoscopic observations allow a reconstruction of the photon direction with
◦
an angular resolution of better than 0.1 . The photon energies can be reconstructed with an
accuracy of 25%.
2 Observations of Supernova Remnants
Since the start of observations with H.E.S.S. 5 shell-type SNRs have been detected in VHE
gamma rays. Four SNRs were known emitters of non-thermal X-rays (RXJ1713.7−3946,
Table 1: Properties of the TeV emission of the shell-type supernova remnants (first part of the table) and
SNR/Molecular Cloud interactions (second part of thetable) detected with H.E.S.S.
name diam. spectral index diff. flux integral flux energy flux
Φ(1TeV) [10−8 I (1−10TeV) F (1−10TeV)
γ γ
TeV−1m−2s−1] [m−2s−1] [ergcm−2s−1]
RXJ1713.7−39465 1.2◦ 2.04±0.04a 21.3±0.5 2.63 10−7 7.5 10−11
RXJ0852−46226 2◦ 2.24±0.04 18.8±0.8 1.4 10−7 5.3 10−11
RCW867 0.9◦ 2.54±0.12 3.6±0.5 2.35 10−8 8.5 10−12
SN10068 0.48◦ 2.35±0.14 0.23±0.04 3.8 10−9 8.3 10−13
2.29±0.18 0.16±0.04
( )
HESSJ1731−3479 0.54◦ 2.32±0.06 4.67±0.19b 1.9 10−8 6.9 10−12
HESSJ1745−30310 ≈ 0.4◦ 2.71±0.11 2.84±0.23 1.6 10−8 5.1 10−12
W2811 mult. 2.49±0.14 1.86±0.19 1.64 10−8 3.5 10−12
2.66±0.27 0.75±0.11
( )
CTB37A12 ? 2.30±0.13 0.87±0.10 6.4 10−9 2.3 10−12
W51C13 ≈ 0.2◦ - - 0.62 10−8c -
RXJ0852−4622, RCW86, SN1006; one SNR (HESS J1731−347) was discovered in the survey
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of the Galactic plane conducted with HESS and was subsequently discovered in radio and
X-rays.
FortheSNRsRXJ1713.7−3946, RXJ0852−4622andHESSJ1731−347theTeVmorphology
is clearly a shell-like structure, which is statistically preferred over a centre-filled morphology.
The morphology of SN1006 are of two spots which are consistent with bright spots of non-
thermal X-ray emission on the SNRs shell. For RCW86 a shell-like morphology could not yet
been confirmed being statistically significant, further observations may clarify the morphology.
The first part of Table 1 lists the SNRs detected with H.E.S.S. with their references and mor-
◦ ◦
phological andspectralparameters. Thediameters rangefrom0.5 upto2 . Theenergyspectra
follow straight power laws, only for RXJ1713.7−3946 an exponential cut-off around 18TeV has
beendetected. Thespectralindicesarebetween 2.0and2.5, whichis consistent withpredictions
from DSA. The detection of gamma-rays from SNRs is evidence for particle acceleration in the
SNR shells.
Another proof of particle acceleration in SNRs are the detection of gamma rays from the
interactions of SNRs with molecular clouds (MCs). Relativistic protons escape the SNR and
interact with the material in a nearby molecular cloud. The high density in the cloud enhances
the gamma-ray emission, and the gamma-ray morphology is in general correlated with the
morphology of the MCs as traced by radio observations of CO emission lines. The second part
of Table 1 summarisesthe objects detected with H.E.S.S.Themorphologies arerather irregular:
the extension of CTB37A is not statistically significant, several clouds are seen around W28,
and the diameters of the other sources are several tenth of a degree. The spectra follow straight
power laws but are in general softer than those seen from shell-type SNRs. The spectral indices
can go up to 2.7, similar to the index of the Cosmic Rays measured directly.
aexponentialcut-off at 17.9±3.3TeV
bat 0.783TeV
ccurrently only theintegral flux is published
Table 2: Leptonic and hadronicinterpretation of theemission of shell-typesupernovaremnants.
name X-ray flux F magnetic field total energy in protons
X
(0.5−10keV) leptonic shock front hadronic scenario
[ergcm−2s−1] B in fractions of E ≈ 1051erg
lep SNR
RXJ1713.7−3946 5.5 10−10d14 ≈10µG ≥ 65µG15 0.1...0.3 d 2 n −1
1kpc 1cm−3
RXJ0852−4622 9.9 10−1116 ≈ 6µG 270µG17 0.08 (cid:16) d (cid:17)2(cid:16) n (cid:17)−1
750pc 1cm−3
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RCW86 2.1 10−10e7 15...30µG 24µG 0.2...0.4 (cid:16)d 2(cid:17) (cid:16) n (cid:17)−1
≈100µG19 2.5kpc 0.7cm−3
SN1006 1.1 10−10f 8 ≈30µG ≥120µG19 0.2 (cid:16) d (cid:17)2 (cid:16) n (cid:17) −1
2.2kpc 0.085cm−3
HESSJ1731−347 3.7 10−11g9 25µG - 0(cid:16).2 d(cid:17) (cid:16)2 n (cid:17)−1
3.2kpc 1cm−3
(cid:16) (cid:17) (cid:16) (cid:17)
3 Interpretation
The fact that the gamma-ray emission from MCs is correlated with the distribution of the
target material for hadronic interaction and the absence of non-thermal X-ray emission from
these sources is a good indication that the emission is indeed from relativistic protons. It can
be shown that the Cosmic Ray density is enhanced with respect to the density in the solar
neighbourhood. And the nearby SNR which is in interaction with the MC is a very good
candidate for being the sources of these Cosmic Rays.
Fortheshell-typeSNRsthesituationislessclearasthesynchrotronemissionhastobetaken
into account. Synchrotron emission from the shells is evidence for a population of relativistic
electrons. These electrons produce gamma-ray emission in inverse Compton (IC) scattering
off the Cosmic Microwave Background (CMB) and the observed gamma-ray emission could be
entirely due to IC emission. Assuming that synchrotron and IC emission are produced by the
same electron population and that the gamma-ray emission is entirely dueto IC emission off the
CMB the magnetic field in the SNR shell can be estimated from the gamma-ray energy flux F
γ
and X-ray energy flux F as B = 10FX µG. As shown in Table 2, the magnetic fields for
X lep Fγ
the shell-type SNRs estimated solelyqfrom the X-ray and gamma-ray fluxes are between 6 and
30 µG. A method to measure the magnetic field in a shock front is from the width of very thin
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filaments observed in X-rays . The magnetic fields estimated from the shock front filaments
are found to be of the order of several 100 µG, significantly larger than what was estimated for
a purely leptonic scenario and consistent with expectations for magnetic field amplification as
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a result of very efficient acceleration of nuclear Cosmic Rays . A higher magnetic field renders
the synchrotron emission more efficient and the observed X-ray emission can be produced by a
smaller electron population. The IC emission is therefore fainter and the gamma-ray radiation
can be attributed to a possible hadronic scenario. Depending on the distance to the SNR and
the density of the ambient material and assuming a certain spectral shape of the underlying
proton population (typically a straight power law with index 2 between 1 GeV and 100 TeV)
thetotal energy inprotonscan beestimated. Thisestimation for theSNRsdetected by H.E.S.S.
is summarised in Table 2 for typical values of the distance d and the density n of the ambient
material. It can beseen that the total energy in protons is of the order of several tens of percent
d1−10keV
e0.7−10keV
f0.5−10keV
gfor roughly 1/3 of theshell
of an assumed supernova explosion energy of 1051erg.
4 Conclusion
Observations with H.E.S.S.have revealed VHE gamma-ray emission from 5shell-typeSNRs and
4 interactions of SNRs with molecular clouds. In the case of the SNR/MC interactions the good
correlation with the distribution of ambient material and the absence of synchrotron emission
id a very good indication for hadronic interactions. The observed magnetic field amplification
in the shells of supernova remnants seem to contradict a purely leptonic scenario and favour
a hadronic scenario. SNRs are therefore very good candidates for acceleration of the Galactic
Cosmic Rays. The extension of H.E.S.S. with a fifth telescope currently under construction will
lower the energy threshold, allowing better studies of the energy spectra of the SNRs. Future
instruments like the Cherenkov Telescope Array (CTA) will have a better angular resolution
and a ten times better sensitivity, enabling detailed morphological studies and surveys for SNRs
throughout the Galaxy.
Acknowledgments
The support of the Namibian authorities and of the University of Namibia in facilitating the
construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the
German Ministry for Education and Research (BMBF), the Max Planck Society, the French
Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of
theCNRS,theU.K.ScienceandTechnology Facilities Council(STFC),theIPNPof theCharles
University, the Polish Ministry of Science and Higher Education, the South African Department
ofScienceandTechnologyandNationalResearchFoundation, andbytheUniversityofNamibia.
We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg,
Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the
equipment.
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